Phosphorus-containing polymers from THPS. VII. Synthesis of phosphorus-containing trialkynes and their metal-free 1,3-dipolar cycloaddition reaction with azidated soybean-oil

Article abstract of DOI:10.1039/c5gc01919b

A novel hydrolysis-resistant trialkyne, namely tris(prop-2-yn-1-yloxy methyl)phosphane oxide (TPOPO) was prepared from an environmentally friendly tetrakis(hydroxymethyl)phosphonium sulfate (THPS). The chemical structure of TPOPO was characterized by FT-IR, NMR (¹H, ¹³C, ³¹P) and MS. The TPOPO underwent metal-free and solvent-free 1,3-dipolar cycloaddition reaction with azidated soybean oil (AzSBO) at different alkyne/azide molar ratios. The gel contents of the cured polymers were higher than 95% and the char yields of polymers at 850 °C were over 16.4%. DMA analysis showed that the glass transition temperatures were over 45.9 °C. The cured polymers showed high dielectric constant and dielectric loss due to their high polarity.

Full text of DOI:10.1039/c5gc01919b

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Phosphorus-Containing Polymers from THPS. VII. Synthesis of
phosphorus-containing trialkynes and its metal-free 1,3-dipolar
cycloaddition reaction with azidated Soybean-oil
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Gang Guo ，Jian Sun , Chen Zhao *, Yun Liu , Cheng-Mei Liu
www.rsc.org/
Abstract: A novel hydrolysis-resistant trialkynes, namely tris(prop-2-yn-1-yloxy methyl)phosphane oxide(TPOPO) was
prepared from an environmental friendly tetrakis (hydroxymethyl) phosphonium sulfate (THPS). The chemical structure of
1
13 31
TPOPO was characterized by FT-IR, NMR ( H, C, P) and MS. The TPOPO underwent metal-free and solvent-free 1,3-
dipolar cycloaddition reaction with azidated soybean oil (AzSBO) with different alkyne/azide molar ratio. The gel contents
of the cured polymers were higher than 95% and the char yields of polymers at 850℃ were over 16.4%. DMA analysis
o
showed the glass transition temperatures were over 45.9 C. The cured polymers showed high dielectric constant and
dielectric loss for their high polarity.
vegetable oil based materials, like many other organic
polymeric materials made from fossil oil, are inherently
5, 6
Introduction
flammable
. Direct utilization of these bio-polymers as
As a class of eco-friendly and annually renewable resources,
vegetable oils have attracted considerable attention as raw
materials for many kinds of polymers which once originated
engineering materials, construction materials, packing
materials and so on, will be confronted with fire threat. The
common strategies of overcoming this demerit are using flame
retardant additives or copolymerizing with flame retardant
monomers in the industry. To meet the application standard,
halogen-based organic flame retardants have been used
widely in industry to prepare nonflammable polymer materials.
However, the flame retardant materials that contain halogens
release hydrogen halides and dioxins during combustion,
1
-13
from fossil oil.
effort is mainly related to the renewability, sustainability,
availability, cost-effectiveness, nontoxicity and
The main driving force for this elaborate
biodegradability of vegetable oil-based polymers. But the
vegetable oils can’t be used directly as engineering materials
for most of them are low molecular weight liquids. Converting
the oil feedstock to serviceable solid materials by relatively
economic and green method is one of the most important
tasks. Though nearly all vegetable oils contain double bands in
their structures, only a few of them can be directly
polymerized into cross-linked polymers because the naturally
occurring double bands in vegetable oil always show low
1
6-20
which will cause corrosion and toxicity.
The concept of sustainable development requires the
development of new green flame retardant polymeric
materials. Many attentions have been paid to phosphorus,
silicon, nitrogen and boron-containing flame retardant
polymers for they generally give off nontoxic combustion
1
4, 15
reactivity.
Most of vegetable oils often require introducing
2
1-24
products during burning
. In fact halogen–free fire
more reactive functional groups into their structures before
polymerization. Many highly reactive oil derivatives have been
successfully prepared by introducing different polymerizable
retardants are increasingly becoming more popular than their
halogen counter parts. By now there are some reports
concentrated on the preparation and characterization of
vegetable oil based fire retardant materials by incorporating
with phosphorus and other elements to improve fire standard
1
, 2, 4, 6, 7
groups
.
Commercially, the inherently safety of polymers is one of the
most important issues which should be considered before
practical application. One of the merits of vegetable oil based
polymers normally is nontoxic, or at least very low toxic. But
2
5
26
of resulting biopolymers . Cάdiz reported two different
approaches to the creation of phosphorus-containing soybean-
oil copolymers. One is the cationic copolymerization of
soybean oil with phosphorus-containing comonomer and the
other one is the cross-metathesis reaction of soybean oil with
functional phosphorus-containing compounds. The resulting
thermosets with 1% phosphorus had LOI values about 24.0 and
indicated an improvement in the fire-retardant properties of
2
7
the soybean-oil-based copolymers. Karger-Kocsisc found that
acrylated epoxidized soybean oil , which was chemically
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grafted with difunctional flame retardants, could
copolymerized with styrene to produce biofoams with high
mechanical properties and intrinsic flame resistance.
Experimental
Materials: Tetrakis (hydroxymethyl) phosphonium sulfate
THPS, 75% solution was purchased from Hubei Xingfa
2
8
（
）
Güngöra prepared Organic–inorganic hybrid UV-curable
coating based on methacrylated/phosphorylated epoxidized
soybean oil. The coating was applied on polycarbonate
substrates and TGA results showed an enhancement in the
thermal stability and also flame retardant properties of the
Chemical Company, China; propargyl bromide, 80% in toluene,
stabilized with MgO, were purchased from Alfa Aesar Reagent
Co., Ltd; Epoxy soybean oil(ESBO), epoxy value 0.408mol/100g,
obtained as gift from Jiangsu Yigete Chemical Co. Ltd, China;
Other chemicals and solvent, such as sodiun azide, fluoroboric
acid, benzyltriethylammonium chloride, N-methylimidazole,
Dimethyl sulfoxide (DMSO), N,N-Dimethylformamide (DMF),
ethyl aceteate, chloroform, were all gained from Sinopharm
Chemical Reagent Co., Ltd., China.
2
9
coated material. Cάdiz
reported that silicon-containing
soybean-oil-based copolymers were prepared from soybean oil
and p-trimethylsilylstyrene by cationic polymerization using
boron trifluoride etherate as initiator. Thermosets with T
ranging from 50 to 62 °C and LOI values from 22.6 to 29.7 were
g
3
0, 31
obtained. They
oil based copolymers from soybean oil and 4-vinylphenyl
boronic acid by cationic polymerization. Thermosets with T
also prepared boron-containing soybean
Characterization and measurements
g
1
The structures of all compounds were verified by proton ( H),
o
ranging from 43 to 60 C and LOI values from 23.7 to 25.6 were
obtained. Bromide was also introduced into vegetable oil by
13
31
carbon ( C) and phosphorus ( P) nuclear magnetic resonance
spectroscopy (NMR) using Bruker AV400NMR spectrometer at
proton frequency of 400 MHz as well as the corresponding
carbon and phosphorus frequencies at room temperature
using deuterated solvents as the solvent. Signals were
3
2
Kusefoglu to improve the flame retardancy of bio-polymers.
But this method suffers from toxicity problem as mentioned
above.
At present, the main method to combine the fire retardant
elements with vegetable oil is cationic copolymerization or
free radical copolymerization of plant oil with functional
comonomers. As we know, the cationic polymerization was
always carried out under harsh conditions and these functional
comonomers (both for cationic and radical polymerization)
can’t be easy obtain on a large scale. So efforts must be paid to
find out more convenient and efficient method to improve the
flame retardancy of vegetable-oil based materials.
1
31
averaged from 256 transients for H NMR and P NMR.
Fourier transform infrared (FTIR) spectra were recorded with a
Bruker Vertex 70 FTIR spectrometer in the region of 4000-400
−1
−1
cm with a resolution of 4 cm . Electrospray ionization mass
spectrometric analysis was carried out using an Agilent 1100
Series LC/MSD Trap XCT. Thermal transitions were monitored
with a differential scanning calorimeter (DSC), Model 204F1
from NETZSCH Instruments, and scan rate of 2-20
a temperature range of 30-300°C and nitrogen flow rate of
0mL/min were used in DSC experiments. Thermogravimetric
℃/min over
3
3, 34
Phosphorus-nitrogen synergistic effect
is a widely
2
accepted concept in the field of fire retardant. To the best of
our knowledge, few reports concerned this topic in the field of
analysis (TGA) was performed with a NETZSCH Instruments’
High Resolution STA 409PC thermogravimetric analyzer that
was purged with nitrogen at a flow rate of 70mL/min. A
heating rate of 20°C/min was used and scanning range was
from 40°C to 900°C. Mechanical properties were measured
using a dynamic mechanical thermal analysis (DMA) apparatus
2
vegetable-oil-based biopolymers by now . In fact, both
7
2
6, 35
phosphorus
and nitrogen elements have been introduced
into vegetable oil by chemical bonding separately. The most
efficient method of preparing nitrogen-containing soybean oil
was to prepare azidated soybean oil (AzSBO) by ring-opening
nucleophilic addition of sodium azide to the epoxy soybean oil
(
PerkinElmer, Diamond DMA). Specimens (50×10×1.0mm)
were tested in a 3 point bending mode. The thermal
transitions were studied in the scope of 20-200 at a heating
3
4, 36-38
.
Then azidated soybean oil can undergo azide−alkyne
℃
cycloaddition with various phosphorus-functionalized alkynes
to form rigid materials containing both phosphorus and
nitrogen elements.
rate of 4°C/min and at a fixed frequency of 1Hz. The contact
angles of the polymer samples after curing were measured at
2
5 °C using a POWEREACH JC2000C1 contact 15 angle
In this paper, a novel phosphorus-containing trialkynes,
goniometer interfaced with image capture software after
injecting a 5-μL liquid drop. To obtain reliable contact data, at
least three droplets were measured at different regions of the
same piece of sample, with at least two pieces of sample being
used. Deionized water was used as a standard when measuring
the surface properties. Dielectric constant and dielectric loss
were measured at room temperature in an air atmosphere by
the two parallel plate modes at 125Hz-1MHz using Agilent
namely
tris(prop-2-yn-1-yloxy
methyl)phosphane
oxide(TPOPO), was first prepared in our group. The key
starting material, tetrakis (hydroxymethyl) phosphonium
sulfate (THPS), has been proved that it is an eco-friendly
chemical and was awarded the American Green Chemicals
3
9, 40
Award in 1997
. The TPOPO underwent thermal Huisgen
1-46
4
cycloaddition reaction
with AzSBO under catalyst-free and
solvent-free environment and obtained both phosphorus-and
nitrogen-containing thermosets. Physical properties such as
glass-transition temperature (Tg) and decomposition
temperatures of these biopolymers are discussed and
compared.
4
2
294A Precision Impedance Analyzer. A sample (about
0mm×10mm×2mm) was placed between two copper
electrodes to form a parallel plate capacitor. Prior to each
measurement, the sample was dried under vacuum at 100 °C
for 3h.
2
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Preparation of tris(prop-2-yn-1-yloxy methyl)phosphane
oxide(TPOPO)
Preparation of cross-linked biopolymer by catalyst-free thermal
cycloaddition reaction
7g (0.05mol) THPO(tris(hydroxymethyl)phosphine oxide) and Three cross-linked biopolymers were prepared in this research
0.7g benzyltriethylammonium chloride (10% weight percent of with different alkyne/azide ratios. The general procedure was
THPO) were added into a four-neck flask equipped with as following. Weighted amount of AzSBO and TPOPO( 15.0g in
condenser, dropping funnel, mechanic stirrer and total) were mixed in a round–bottomed flask equipped with
thermometer. 20mL of 30% NaOH aqueous solution was magnetic stirrer, 10mL chloroform was added into it to dilute
dropped into the flask at such speed that the temperature of the mixture. The mixture was stirred at room temperature for
o
reaction mixture was below 35 C under mechanical stirring. 0.5h to get a well-mixed solution. Then the solvent was
After finishing addition of NaOH solution, the temperature of stripped off by rotatory evaporator under vacuum. The
mixture was cooled to 0
℃
by ice-water bath. 29.0g (80wt% in mixture
was
transferred
to
aluminum
mould
toluene, 0.195mol) propagyl bromide was added dropwise (50mm*10mm*1mm), and the air was removed at 60 °C and
under stirring in 2h and the temperature was controlled below 80 °C each for 2 h under vacuum, The step curing was achieved
o
during addition. After all propagyl bromide was added to in an oven at 100 °C for 12 h, then at 130 °C , 160 C and 190 °C
5℃
the flask, the temperature was raised up to 50
℃ and kept at each for 2 h. The sample was cooled down to room
this temperature for 10h under rapid stirring. The reaction temperature slowly and kept at this temperature at least 48h
mixture was cooled to room temperature and suitable amount before next measurements.
of water was added in order to dissolve the NaBr. Then the
mixture was transferred into separation funnel and the organic
-
layer was washed with water for several times until no Br ion
Results and discussion
being detected by AgNO solution. The organic layer was dried Synthesis and characterization of starting chemicals
3
with anhydrous Na SO . After filtration, the remaining
2
4
The multiple-alkyne compounds are key starting materials in
dipolar cycloaddition reaction. Though many kinds of alkynes
have been used in click chemistry, phosphorus-containing
multiple-alkynes were seldom reported in literatures according
propagyl bromide was distilled out at 90-95
℃ under normal
pressure. Finally, the remaining organic liquid was placed in
high vacuum to remove all volatiles. 7.94g brown viscous liquid
was obtained. The yield based on THPO was 62.9%. MS:
to the best of our knowledge. Tripropargyl phosphate and
47-49
+
m/z[M+Na] = 277.05947.
analogous compounds
are readily obtained phosphorus-
containing compound, but its hydrolysis-sensitivity and strong
acidity after hydrolysis will impose negative effect on final
Synthesis of Azidated Soybean Oil (AzSBO)
materials. So
containing
a
novel hydrolysis-resistant phosphorus-
Azidated soybean oil-based (AzSBO) was synthesized via a ring-
opening reaction of epoxy groups of ESBO with sodium azide
trialkynes, namely tris[(prop-2-yn-1-
yloxy)methyl]phosphane oxide(TPOPO), was first designed and
synthesized in our group according to Scheme 2.
+
−
using 1-methylimidazolium tetrafluoroborate([Hmim] BF
the catalyst according to the previously reported procedure .
ESBO (183.8 g, 750 mmol epoxy), NaN (73.9 g, 1.125mol),
(27.8 g , 15.1wt%), and DMF (170 g) were added
4
) as
3
8
3
+
−
[
Hmim] BF
4
into a 500 mL one-neck round bottom flask equipped with a
condenser, mechanic stirrer, and oil bath. The reaction mixture
was stirred at 95 °C for 60 h. At the end of reaction (60 h), the
reaction mixture was allowed to cool to room temperature,
dissolved in ethyl acetate (800mL), and transferred to a
separatory funnel. The solution was successively washed with
saturated sodium chloride solution (100 mL*5) and distilled
water (100 mL*5) to remove the catalyst and unreacted
sodium azide. The organic layer was dried over magnesium
sulfate and the solvent was removed under vacuum at 40°C.
AzSBO was obtained as a dark-brown and viscous liquid with
81.3% yield. The nitrogen content of the AzSBO was
determined by elemental analysis.
Scheme 2 Synthesis of TPOPO and its thermal cycloaddition
reaction with AzSBO
The starting tetrakis(hydroxymethyl) phosphonium sulfate
(THPS), is an eco-friendly water-soluble chemical which can be
Scheme 1 Reaction of ESBO with NaN
soybean Oil (AzSBO)
3
to prepare azidated obtained at industrial scale in China as main co-product during
sodium hypophosphite production. THPS was originally used as
a biocide in industrial cooling systems, oil field operations, and
the paper-making industry and it is readily biodegradable and
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1
has no potential to bioaccumulation. Meanwhile THPS is
H NMR spectrum of TPOPO is shown in Fig.3. The peak locates
The
-O-;
rapidly oxidized
in
the
environment to
THPO at 4.30ppm originated from the two protons in ≡C-CH
-；
2
(
trishydroxymethylphosphine oxide) which has a very low peak appears at 4.08ppm are attributed to protons of P-CH
2
aquatic toxicity and is not considered to present an peak centered at 2.53ppm are assigned to protons of -C≡CH.
environmental hazard. So, THPS passed an environmental The integral peak area ratio of three kinds of protons is 2:2:1,
attestation in America and was awarded the American Green exactly equals to theoretic value.
3
9
Chemicals Award in 1997
.
The THPS was nearly quantitatively transferred to THPO under
basic condition and air was used as the oxidant. TPOPO was
prepared by standard Willamson method with the aid of phase
4
0
transfer catalyst (PTC) . That is, transformation of hydroxyl
group into sodium alkoxide with sodium hydroxide and
successive treatment with propagyl bromide. The TPOPO was
obtained as brown viscous liquid and the yield of TPOPO was
over 60% based on THPO. The concentration of NaOH was in
the range of 25-30% (w/w) for higher concentration of NaOH
would result in highly viscous reaction mixture which hider the
reaction going smoothly. On the contrary, lower concentration
of NaOH would lower the reaction rate and product yield.
1
The FT-IR spectrum of TPOPO is shown in Fig.1.The Fig. 3 H NMR spectrum of TPOPO
characteristic peaks of -C≡C- stretch vibration appears at
-
1
13
2
114.1cm , and the vibration absorption of ≡C-H locates at
-1
C NMR spectrum of TPOPO is shown in Fig.4. The signal at
288.4cm , 3213.3cm and 641.5cm . The typical peak for 78.23ppm originated from the carbon of -C≡CH and at
-1
-1
3
-
1
P=O appears at 1180.3cm and the peak for C-O-C at 76.19ppm generated from the carbon of -C≡CH. The peaks at
-1
-1
-1
1
092.7cm . The peaks appearing at 2895.8cm
、2858.8cm
63.28ppm~64.27pm are attributed to the methylene carbon
connecting to phosphorus atom (P-CH -O), the signals at
0.71ppm~60.59ppm are due to the methylene carbon atom
are due to -CH -vibration.
2
2
6
2
of -CH -C≡CH .The peaks located in the range of 77.54ppm~
7
6.90ppm are assigned to solvent.
Fig. 1 FT-IR spectrum of TPOPO
3
1
The P NMR spectrum of TPOPO is showed in Fig 2. It is very
clear that only one peak appears at 39.29ppm. This result is
exactly consistent with the theoretic structure of TPOPO.
1
Fig. 4 C NMR spectrum of TPOPO
3
3
8
AzSBO was prepared according to reported method . The FT-
IR spectra of ESBO and AzSBO are depicted in Fig.5. Obviously
-
1
-1
，and 2103.9 cm (-
two new peaks appears at 3376.3 cm (OH)
N
3
). The former peak is attributed to hydroxyl groups
originated from the ring-opening reaction of epoxy group, the
next one from the azide groups. The characteristic peaks for
-1
epoxy group at 825.5cm disappeared completely. The
nitrogen content of the AzSBO was determined by elemental
analysis and the data listed in Table 1.The average nitrogen
3
content is 11.03% and it means that the per -N group equals
3
Fig. 2 P NMR spectrum of TPOPO
1
382.1g of AzSBO. The sulfur came from the sulfonated product
4
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of soybean oil. For concentrated H
preparing the ESBO.
2
SO
4
was the catalyst for To better understand the curing processes, the curing kinetics
was investigated using non-isothermal differential scanning
calorimetry (DSC) at different heating rate (Fig. 6). Kissinger,
5
0-52
Ozawa, and Flynn–Wall–Ozawa methods
were used to
determine the kinetic parameters and built the kinetic models.
-1
The activation energy for preparing P1 is 69.0 KJmol (Kissinger
-1
-1
method), 72.3 KJ mol (Ozawa method), 74.5 KJ mol (Flynn–
Wall–Ozawa method), respectively. The reaction is a typical
self-catalyzed reaction and the calculated curing parameter at
o
o
o
heating tare 0k/min is 92.65 C(T
o p t
), 127.00 C(T ), 164.18 C(T ),
respectively. So the curing procedure for preparing sample was:
o
00 °C curing for 12 h, then at 130 °C, 160 C and 190 °C each
1
for 2 h. The post treatment at high temperature can improve
the percent conversion of alkyne and azide.
Fig. 5 FT-IR spectra of ESBO and AzSBO
Table 1 The result of elemental analysis for AzSBO
Number
Weight[mg]
N[%]
C[%]
H[%]
S[%]
1
2
2.4250
2.6850
11.04
11.01
62.04
62.04
9.263
9.485
0.441
0.352
average
11.03
62.04
9.374
0.397
Thermal Huisgen reaction studied by DSC
Fig. 6 DSC thermograms of TPOPO with AzSBO with different
Three crosslinked polymers with different alkyne/azide ratios heating rate
were prepared according to Table 2. Before curing reaction,
o p t
the thermal Huisgen cycloaddition reaction was studied by Table 3: The date of T , T and T for polymer P1 at the
differential scanning calorimetry (DSC) at different heating rate different heating rate
(
P1 as example). The results of DSC study are shown in Fig. 6 Heating rate
T
o
T
p
T
t
o
and the corresponding data are collected in table 3. With K( C)/min
o
o
o
K ( C)
368.05(94.90)
K( C)
K ( C)
increasing the heating rates
o
of cycloaddition reaction increased from 94.9 C to 125.8 C,
，
the initiation temperatures(T )
o
2
399.95(126.80）
415.35(42.20）
428.25(155.10）
436.55(163.40）
445.65(172.50）
400.15(127.00）
436.65(163.50）
o
the peak temperatures(T ) of exothermal process of reaction
p
5
375.25(102.10)
383.65(110.50）
397.45(124.30）
398.95(125.80）
365.80(92.65）
454.75(181.60）
470.25(197.10）
479.65(206.50）
488.95(215.80)
437.33(164.18)
o
shifted from 126.8 C to 172.5 C and the termination
o
o
temperature of reaction varied from 163.5 C to 206.5 C.
o
10
15
20
Table 2 Formulas for crosslinked polymers and related
properties
Polymer
P1
P2
P3
AzSBO/TPOPO
Weight(g)
12.28/2.73
12.03/2.97
11.75/3.26
0
(
Calculated)
AzSBO/TPOPO
4.50
1:1
4.05
1:0.9
2.42
8.84
3.60
1:0.8
2.65
8.63
(
Weight ratio)
Alkyne/azide
molar ratio)
Thermal Huisgen reaction studied by FT-IR
(
The curing reaction of AzSBO and TPOPO was also monitored
by FT-IR and the spectra are shown in Fig.7. At room
Phosphorus
2.22
9.02
Content (%,wt)
Nitrogen content
-1
temperature, the strong absorption peaks at 2106.2cm
originated from both azide group and triple bond. After curing
(
%, wt)
o
-1
at 100 C for 12h, the peak at 2106.2cm disappeared almost
completely. Further heat treatment has no notable effect on
peak intensity. It indicated that the newly prepared trialkyne
Gel Content(%)
95.0
96.0
95.0
o
Contact angel( )
80.6°
83.4°
87.4°
-1
could react with AzSBO smoothly. The peaks at 3310.9cm and
-
1
6
48.1cm (due to triple bond) also weakened with heat
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treatment. The characteristic vibration peak for triazole group gentle indicating the formation of thermal stable char. The
-1
appeared at 3153.4cm after curing reaction. During the fire-retardant property of polymer can be estimated from
thermal Huisgen cycloaddition reaction, the typical peaks of limited oxygen index (LOT) which can be calculated according
-
1
-1
40
C=O (1737.7cm ) and P=O(1093.9cm ) kept unchanged and to the empirical formula :
indicated the high thermal stability of cured polymer.
LOI=17.5+0.4Yc,
is the char yield of polymer at 850 C in nitrogen
atmosphere.
o
where Y
c
Fig. 10 TGA curves of polymers under Nitrogen
Fig. 7 FT-IR spectra of TPOPO with AzSBO under various
curing condition
The char yields of three polymers are 16.4%, 16.4% and 18.6%
respectively. The calculated LOI values are 24.1, 24.1 and 24.9,
respectively. Obviously the flame retardancy of soybean oil-
based polymers improves dramatically by introducing
phosphorus and nitrogen elements. Comparing with previous
，
Thermal stability of crosslinked polymers
According to presetting curing procedure, three crosslinked
polymers were prepared for property study. All crosslinked
polymers show dark red appearances (Fig. 8) and can be bent
to some extent. The DSC curves of cured polymers are
depicted in Fig. 9 and it is found that there is no extra
endothermal or exothermal peak appearing, which means that
the cycloaddition reaction between alkyne and azide nearly
completed. Though unreacted alkyne groups may remain in P2
and P3, but the total amount of remaining alkynes was small
and the polymerization enthalpy of remaining alkyne was too
weak to be recorded by DSC instrument.
3
4, 36, 37
reports
, the char yield of most AzSBO-based polytriazole
o
o
was lower than 15% at 600 C and no data was given at 850 C,
this research provides a convenient and effective method to
improve the fire-rertardancy of soybean oil-based thermosets.
Table 4 Thermal properties of three cured polymers
T
Polymers
T
5%
T
10%
T
max1
max2
Y
c
LOI
(℃)
(℃)
(℃)
(℃)
(%)
P1
278.4
321.1
366.8
461.7
16.4
24.1
P2
P3
276.1
289.1
319.2
332.3
370.4
376.9
440.9
477.3
16.4
18.6
24.1
24.9
T
T
T
Y
5%: Temperature of 5% weight loss.
10%: Temperature of 5% weight loss.
max: Temperature at Maximum decomposition rate.
o
: Char yield at 850 C under nitrogen.
c
LOI: Limit Oxygen Index.
It is also noticed that the molar ratio of Alkyne/azide has some
effect on thermal properties of three polymers. Analyzing the
data listed in Table 4, it seems that the P3 shows a little better
thermal stability than that of P1 and P2.The possible
explanation is the excess alkyne will provide extra crosslinking
sites at high temperature, resulting in improvement of thermal
stability. Meanwhile the phosphorus content of P3 is slightly
higher than that of P1 and P2. It is well known that phosphorus
Fig. 8 Digital Photographs of Fig. 9 DSC curves of three
P1,P2 ,P3
crosslinked polymers
The thermal stability of three crosslinked polymers was
measured by thermogravimetric analysis. Both TG and DTG
curves are shown in Fig 10 and corresponding data are
collected in Table 4. From TG curves, we can find that three
2
2, 23
o
can promote the formation of carbon layer
.
polymers showed high thermal stability below 300 C and
The dynamic mechanical properties of the step cured polymers
were obtained as a function of the temperature beginning in
the glassy state of each sample to the rubbery plateau of each
weight loss is less than 10%. The rapid weight loss of three
o
polymers appears in the temperature range of 300 C to 520 C,
o
o
the first maximum weight loss temperature locates at 375 C
o
and the second one appears at about 450 C as DTG curves
material. The T , storage moduli and cross-linking density of
g
o
show. Between 600 C to 900 C, the weight loss rate becomes
o
the three polymers can be deduced from the corresponding
curves (Fig. 11) and all data are listed in Table 5.
6
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ARTICLE
Tanδ: dielectric loss at 1MHz at room temperature.
Surface properties
The surface properties of three polymers were studied by
measuring the water contact angle (Table 2) .The contact
angles are 80.6°(P1), 83.4°(P2), and 87.4°(P3), respectively.
5
4-57
Comparing with reported results
, the present three
polymers show a little stronger hydrophilicity. This result is
attributed to the fact that phosphorus is a high electronegative
element and P=O bond has a strong tendency to form
hydrogen bond with water. Meanwhile the surface property
also relates to chain segment movement. For Polymer P3, it
possesses largest contact angle among three polymers, the
reason is that the chain movement of P3 is easier than that of
other two polymers for its low crosslinking density. As we
know, the long alkyl chains in soybean oil are typical
hydrophobic. At low crosslinking density, some of them can
move to the sample top and form a hydrophobic surface. But
in polymer with high crosslinking density, the chain movement
is restricted to some extent for lack of enough free volume,
such as in P1.
Fig. 11 DMA curves of polymers
o
s of the three polymers are 57.0 C, 53.0 C,
o
It is found that the T
g
o
5.9 C, respectively. With increasing of alkyne/azide molar
4
g
ratio, the T of corresponding polymer declines slowly. For at
high molar ratio of functional group, the unreacted alkyne
component can act as plasticizer to lower the glass transition
temperature of polymer. The storage modulus of three
polymers show same changing trend as T
temperature, the storage moduli of P1 is 1.02 GPa, however, it
is 0.97GPa for P2 and 0.70GPa for P3, respectively. Normally T
g
s. At room
g
and mechanic properties of crosslinked polymers mainly
depend on crosslinking density, high crosslinking density
normally results in more stiff polymer. The cross-linking
Dielectric properties of cured polymers
Dielectric property of polymers is very sensitive to its polarity,
5
additives and impurity remaining during production . The
dependence of the dielectric constant and dielectric loss of all
8
e
density (υ ) is the number of moles of network chains per unit
volume of the cured polymers and it can be estimated from
the rubbery plateau modulus using an equation from the
samples in the frequency range of 400Hz 1MHz were studied
~
3
4, 53
at room temperature (Fig. 12 and Table 5). Comparing with
5
statistical theory of rubber elasticity theory
=E’/3RT
:
9, 60
previous researches
, both the dielectric constant (ε) and
υ
e
the dielectric loss (tan δ) of present three polymers are bigger
than that of polytriazoles from soybean oil source. This result
is assigned to following reasons. First, as we mentioned above,
P=O bond is a high polarizable group, introducing phosphorus
into polymer network will increase the polarity of final
polymer. Meanwhile, AzSBO contains many hydroxyl groups
where, E’ is tensile storage modulus obtained in the rubbery
plateau, T is temperature in °K corresponding to the storage
modulus value, and R is the gas constant.
Though the equation is strictly valid only for lightly cross-linked
materials, it can also be used to compare qualitatively the level
of cross-linking among structure-likely polymers. The
calculated crosslinking-density of three polymers is shown in
3
3
Table 5, they are 0.39mol/m (P1), 0.19 mol/m (P2),
3
.17mol/m (P3), respectively. Among the three polymers, P3
0
shows the lowest crosslinking density and possess more free
volume to let the chain segment move freely at low
temperature, so its T
g
value is the smallest one.
Table 5 Mechanical and dielectric Properties of Polymers
Polymer
P1
T
g
E
1
′
E
2
′
E
3
′
υ
e1
ε
Tanδ
3
3
(℃)
(GPa) (GPa) (MPa) (10 mol/m )
57.0 4.31
1.02
4.35
0.39
9.4
0.25
Fig. 12 Dielectric constant and Dielectric loss vs frequency of
P2
P3
53.0 1.87
45.9 1.67
0.97
0.70
2.16
1.87
0.19
0.17
8.9
0.10
0.08
polymers
10.0
and azide groups after ring-opening reaction of epoxy with
azide, and they are both polarized easily to result in high polar
product. Moreover, some impurity and byproduct, such as
sulfonated soybean oil, also remain in polymer and the
sulfonic acid groups also have contribution to high dielectric
constant of crosslinked polymer. The elemental analysis results
indicate the presence of sulfur. The electric loss values of three
T
g
1
2
3
: The glass transition temperature.
o
′: the storage modulus at -60 C of glassy states.
E
E
E
υ
′: the storage modulus at room temperature.
o
′: the storage modulus at 175 C of rubbery states.
o
e1: the crosslinking density at 175 C.
: dielectric constant at 1MHz at room temperature.
ε
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ARTICLE
Journal Name
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purity according to structure analysis experiment, the AzSBO
contains many hydroxyl, sulfonic and other polar groups and
these groups will act as charge carrier when voltage is applied
to sample, so it will show high dielectric loss. This result
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and 289.1℃ , respectively,
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Foundation of Hubei Province, China(Grant No. 2015CFB188 ) ,
and Opening Project of Key Laboratory of Optoelectronic
Chemical Materials and Devices of Ministry of Education,
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Graphical Abstract